Designing, constructing, and operating today's AM directional array is becoming increasingly sophisticated and complex. There are increasing demands for new stations and better coverage from increasingly challenging physical locations. Today, there are nearly 5,000 licensed AM stations in the United States. Over 1,800 of these are licensed for directional operation. At this time there are a little more than 500 construction permits outstanding for new and modified directional facilities and over 900 applications for construction permits.
The opportunities for these new and improved facilities are made possible by sophisticated computer programs that can analyze existing licensed station coverage or proposed coverage as well as the FCC rules providing protection for stations on the same frequency and adjacent frequencies, and signal propagation to produce intricate antenna patterns. These patterns can then be analyzed to produce physical antenna array specifications and electrical networks to maximize coverage.
An important part of adjusting and maintaining an AM directional antenna system is accurately monitoring the phase and amplitude of the RF current in each directional element in the antenna array. A conventional method of sampling the phase and amplitude is with a sampling coil or loop positioned off the antenna tower that is connected by a coaxial cable to an antenna monitor. In order to avoid coupling to the base of the antenna tower, the coaxial cable leading to the sampling coil is wound into a high impedance choke. In addition, it is generally necessary to ensure that the coaxial cables that extend from the antenna monitor to the sampling coil on each of the towers are the same length and are subject to the same environmental conditions. This is typically accomplished by burying the coils of coaxial cables underground. The result is often a mass of cables that must be maintained and periodically inspected to ensure that the differences in the signals measured at the antenna monitor are due variations in the current flowing within the antenna towers themselves and not due to differences in the coaxial cables. Given these problems, there is a need for an easier method of determining the current and phase in antenna elements of a directional broadcast antenna.